The present invention relates to methods and apparatus for testing semiconductor lasers and, more specifically relates to methods and apparatus for wafer-level testing of vertical cavity surface emitting lasers (VCSELs).
Semiconductor lasers in use today include edge-emitting diode lasers and vertical cavity surface emitting lasers (xe2x80x9cVCSELsxe2x80x9d). In an edge-emitting laser, a semiconductor gain medium, for example, a quantum-well semiconductor structure, is formed on a surface of a semiconductor substrate. Once a device is detached from a wafer, cavity mirrors are formed or otherwise positioned on opposite ends of the gain medium, perpendicular to the substrate surfaces, to form a resonant cavity within which the gain medium is located. Electrical or optical pumping of the gain medium generates a laser beam which propagates in a direction along the plane of the substrate. As edge-emitting lasers generate a beam in a direction along the plane of a substrate forming the laser, these lasers can not be meaningfully tested in wafer form - that is, it is not practical to test these lasers prior to their being cleaved into individual units exposing the edges from which their beams are output.
VCSELs in contrast, propagate output beams in a direction perpendicular to the plane of a substrate on which they are formed. Thus the orientation of VCSELs on a wafer substrate prior to their being separated from one another is potentially suitable for testing. Prior wafer probe methods used on VCSELs involve electrically probing the optical aperture side of a wafer and detecting light emitted from that side while shorting the opposite side of the wafer to ground. Depending on the resistance profile of the wafer, this method may stimulate emissions from a single VCSEL, or emission from other VCSELs adjacent to and even substantially separated from the VCSEL to be probed due to the low electrical resistance between adjacent ones of the VCSEL array on the probed side.
The present invention is directed to a method for wafer level testing of semiconductor lasers comprising the steps of positioning a wafer on a chuck with a first side of the wafer contacting the chuck including optical apertures through which output beams of the lasers included therein are emitted and electrically probing individual ones of the lasers on a second side of the wafer to stimulate emission from the accessed lasers in combination with the step of detecting light from the accessed lasers after the light has passed through the chuck.
The present invention is further directed to a device for wafer level testing of semiconductor lasers comprising a chuck on which a wafer including lasers to be tested is received, wherein a first side of the wafer contacting the chuck includes optical apertures through which output beams of the lasers included therein are emitted and an electrical probe accessing individual ones of the lasers on a second side of the wafer to stimulate emission from the accessed lasers in combination with a light detector receiving light from the accessed lasers after the light has passed through the chuck.